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New Aspects of Septin Assembly and Cell Cycle Control in Multinucleated A New aspects of septin assembly and cell cycle control in multinucleated A. gossypii Inauguraldissertation zur Erlangung der Würde eines Doktors der Philosophie vorgelegt der Philosophisch-Naturwissenschaftlichen Fakultät der Universität Basel von Hanspeter Helfer aus Selzach, SO Basel, 2007 Genehmigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von Prof. Dr. Peter Philippsen, Prof. Dr. Anne Spang und Prof. Dr. Jean Pieters Basel, 6. Juni 2006 Prof. Dr. Hans-Jakob Wirz Dekan Table of Contents Summary 5 General Introduction 6 Chapter I A conserved cell cycle control module leading to CDK tyrosine phosphorylation functions in the A. gossypii starvation response 9 Introduction 9 Results 11 - Phosphorylation of Cdc28-Y18 is a candidate for regulation of the nuclear cycle 11 - Minimal AgCdc28-Y18 phosphorylation under ideal growth conditions 11 - AgCdc28p is phosphorylated on tyrosine 18 when hyphae are starved for nutrients 11 - The nuclear cycle is delayed in specific stages of division during starvation 12 - Homologues of the yeast morphogenesis checkpoint components in A. gossypii 12 - AgSwe1p is responsible for AgCdc28Y18 phosphorylation 13 - Only minimal role of morphogenesis checkpoint homologues in regulating nuclear density under non-starving conditions 13 - Starvation induced nuclear cycle delay depends on AgSwe1p 13 - Mitosis are concentrated at sites of branch formation 14 - Table 1. Homologues of the yeast MC components in A. gossypii 16 Discussion 17 Chapter II Septins – cytokinesis factors in the absence of cell division in A. gossypii 20 Introduction 20 Results 22 - Septins are conserved in two organisms of most diverse morphology 22 - Continuous AgSep7p-GFP ring at the neck of budding yeast 22 - Intermitted hyphal AgSep7p-GFP rings in A. gossypii 22 - Variety of septin organizations 23 - Chronological order of septin localization 23 - One ring to rule them all 23 - A. gossypii septins are not essential but required for efficient growth and proper morphogenesis 24 - Cortical barrier? 24 - Separating the “twins” (AgCDC11A/B) 25 Discussion 26 T ABLE OF CONTENTS 3 Materials and Methods 29 - A. gossypii methods, media and growth conditions 29 - Plasmid and strain construction 29 - Protein extraction and Western blotting 31 - Fluorescence stainings, image acquisition and processing 31 - Table 2. A. gossypii strains used in this study 33 - Table 3. Plasmids used in this study 34 - Table 4. Oligonucleotide primers used in this study 35 Appendix 37 I Verification PCRs II Plasmid maps III Sequence alignments References 38 Acknowledgments 48 Curriculum vitae 49 T ABLE OF CONTENTS 4 Summary Chapter I Nuclei in the filamentous ascomycete Ashbya gossypii divide asynchronously and most nuclei have the potential to divide (Alberti-Segui et al., 2001; Gladfelter et al., 2006). Although cytoplasmic extension is restricted to growing tips and emerging branches, the distances between nuclei are uniform along the entire hyphal length. This implies active control of nuclear distribution and division to maintain an ideal nuclear to cytoplasmic ratio, potentially depending on environmental conditions. The question of nuclear distribution has already been addressed earlier (Alberti-Segui et al., 2001). We have investigated how the rate of mitosis is regulated in response to intra- and extracellular signals. Here we show that homologues of S. cerevisiae morphogenesis checkpoint components are involved in starvation response in A. gossypii: Phosphorylation of the cyclin dependent kinase AgCdc28p at tyrosine 18 by the protein kinase AgSwe1p is used to delay mitosis under low-nutrient conditions, leading to an increase in the average distance between nuclei. This effect is markedly reduced in Agswe1Δ or Agcdc28Y18F mutants where the CDK cannot be phosphorylated. Overexpression of AgSWE1 leads to decreased nuclear density even under non-starving conditions. Addition of rapamycin mimics starvation response, suggesting that AgSwe1p may be under control of AgTor1/2p. In unperturbed budding yeast cells, ScSwe1p is recruited to the septin ring at the mother-bud neck where it is phosphorylated and subsequently degraded. We have speculated that the septins in A. gossypii could serve as spatial markers to locally inactivate AgSwe1p and increase nuclear division rate in areas of growth. Time-lapse analysis has revealed that mitoses in wild type are most common near branching points. Interestingly, AgSep7p-GFP localizes to branching points and septin deletion mutants show random distribution of mitoses. We propose a model in which AgSwe1p may regulate mitosis in response to cell intrinsic morphogenesis cues and external nutrient availability in multinucleated cells. Chapter II Septins are evolutionary conserved proteins with essential functions in cytokinesis, and more subtle roles throughout the cell cycle. Much of our knowledge about septins originates from studies with S. cerevisiae, where they form a ring-like protein scaffold at the mother-bud neck. We have asked what functions the septins may hold in an organism that does not complete cytokinesis prior to sporulation. Interestingly, all budding yeast septins are conserved in A. gossypii and one is even duplicated (S. Brachat, personal communication; Dietrich et al., 2004). In vivo studies of AgSep7p-GFP have revealed that septins assemble into discontinuous hyphal rings close to growing tips and sites of branch formation and into asymmetric structures at the base of branching points. Rings are made of filaments which are long and diffuse close to growing tips and short and compact further away from the tip. During septum formation, the septin ring splits into two to form a double ring. Agcdc3Δ, Agcdc10Δ, and Agcdc12Δ mutants display aberrant morphology and are defective for actin- ring formation, chitin-ring formation, and sporulation. Due to the lack of septa, septin deletion mutants are highly sensitive, and lesion of a single hypha can have catastrophic consequences for a young mycelium. Strains lacking AgCDC11A show morphological defects comparable with other septin deletion mutants, but actin- and chitin-ring formation are not disabled. Deletion of AgCDC11B results in no detectable phenotype under standard laboratory conditions. S UMMARY 5 General Introduction Filamentous fungi differ in many striking ways genetic functional analysis (Steiner et al., 1995; from common single-celled eukaryotes. The Altmann-Johl and Philippsen, 1996; Mohr, PhD mycelium formed by hyphal growth is a dense, Thesis, 1997; Wendland et al., 2000; Wendland complex network of branched filaments, whose and Philippsen, 2000; Dietrich et al., 2004). size is only limited by external conditions such as availability of resources, environmental The A. gossypii life cycle starts with the only stresses, and competing organisms. Mitosis are known phase of isotropic growth in wild type: either synchronized (all nuclei divide germination of the haploid spore to form a germ simultaneously), or they occur in an bubble. This is followed by apical growth, asynchronous manner (Nygaard et al., 1960; extending two germ tubes in succession on Clutterbuck, 1970). In either way, nuclear opposing sites of the germ bubble. More axes of division is not followed by cytokinesis, leading to polarity are established with lateral branch multinucleated hyphae which share one common formation in young mycelium. Maturation is cytoplasm. Thus, although by appearance such characterized by apical branching and a dramatic mycelia certainly do not correspond to the increase of growth speed (up to 200 μm/h at common perception of a cell, a mycelium formed 30°C, (Knechtle et al., 2003)), which enables it to by true hyphal growth has to be considered as cover an 8 cm-Petri-dish of full medium in about one single, multinucleated cell. Filamentous 7 days. Sporulation is thought to be induced by growth enables these organisms to rapidly cover nutrient deprivation, leading to contraction at the and exploit solid surfaces, and the polarized septa, cytokinesis and subsequent abscission of force generated by apical extension allows them sporangia which contain up to 8 haploid spores to penetrate tissue which would otherwise not be (Figure Intro.1, adopted from Brachat, PhD accessible. However, this growth mode also thesis, 2003) (Ayad-Durieux et al., 2000; bears new challenges: How can one single cell Brachat, PhD thesis, 2003; Knechtle et al., have the flexibility to react to conditions, which 2003). Hyphae are compartmentalized by septa, may greatly differ from one part of the mycelium which in young parts appear as rings that allow to the other? How can it prevent local damage transfer of nuclei (Alberti-Segui et al., 2001) and from spreading through the entire mycelium? in older parts may appear as closed discs. How are hyphal extension and branching Compartments typically contain around eight regulated to ensure optimized growth and nuclei. Nuclear pedigree analyses carried out by resource exploitation? How does such a cell Peter Philippsen and Amy Gladfelter have shown maintain an optimal protein per cytoplasm ratio, that neighboring nuclei are in different division although the cytoplasmic volume may increase cycle stages. Most nuclei have the potential to with various speed in different places of the divide, with the time between two divisions mycelium? varying greatly from 46 – 250 min (Gladfelter et al., 2006). Despite limitation of growth to tips and We sought to study some of these questions in branching sites, distances between
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